In electronic cameras, the light-sensitive elements or pixels convert light incident through an objective of the camera into electric signals. Each of the pixels is addressed to read out an image, with a pixel corresponding to a picture element of the image. A signal which is proportional to a charge of the pixel collected by an exposure is conducted to an output of the image sensor.
Image sensors are in particular known which have a separate row selection line for each row and a separate column line for each column. The reading out of such an image sensor takes place row-wise, i.e. row for row. For this purpose, the pixels of the respective row are switched to the column lines by means of the respective row selection line. A separate column amplifier is associated with each of the column lines to amplify the signals of the pixels of the selected row applied at the column lines. The column amplifiers are in this respect arranged in a row located beneath or above the image field. The amplified signals are conducted via a multiplexer device to the output or—if a plurality of outputs are provided, as is preferred to achieve a high picture rate—to the outputs of the image sensor.
The pixels of the image field are further reduced in size again and again with each new technology generation to increase the resolution of the image sensors. Since the aforesaid column amplifiers usually have approximately the same width as the pixels, the width of the column amplifiers also has to reduce accordingly with the ever smaller pixels.
If this is not possible or is disadvantageous, an image sensor in accordance with FIG. 1 can be equipped with at least one row 21 of column amplifiers 25, in particular two or more such rows 21 arranged at a first side of the image field 11, and with at least one such row 23, in particular two or more such rows 23, arranged at a second side of the image field, with the image field 11 having the pixels 13 being arranged between the first row 21 and the second row 23. The column amplifiers 25 can then be arranged in the row 21 above the image field 11 and in the row 23 beneath the image field 11, and the column lines 17 can then alternately be conducted upwardly and downwardly to the respective row 21, 23 of column amplifiers 25. In such an arrangement, the width of the column amplifiers 24 can be twice as large as the width of the pixels 13. A line addressing logic 19 is also shown in FIG. 1.
In an image sensor which has a plurality of column lines which extend parallel to one another for the respective column, for example two or four column lines, to achieve a higher read-out speed, a corresponding increase in the maximum possible width of the column amplifiers can be achieved.
To achieve a high dynamic range with the image sensor, the pixels and column amplifiers are designed as extremely low-noise and thus, however, also as particularly sensitive. A small signal aliasing or signal difference thereby already results in an aliased or differing brightness of the picture element corresponding to the respective amplified signal. Such interference or such a difference is, however, not perceptible to the eye provided it occurs randomly or only pointwise.
If the signal aliasing or signal difference is, however, caused systematically by one of the column amplifiers, the interference or difference occurs in all pixels of the column of the image sensor associated with the respective column amplifier and thus in all picture elements of the corresponding column of the image so that a vertical stripe arises in the image which can be perceived by the eye. A vertical stripe can arise independently of whether one or more rows of column amplifiers is/are provided or whether, with a plurality of rows, they are arranged on the same side or—distributed in whatever manner—are arranged at different sides of the image field.
In an image sensor having the explained two rows of column amplifiers, the electric properties of the column amplifiers of the upper row such as offset voltage and amplification as a rule differ at least slightly from the column amplifiers of the lower row. This difference is caused by the fact that the named properties of the column amplifiers are dependent on different influencing variables such as temperature, voltage supply and production tolerances and thus have a relatively high location dependency. Such differences are in this respect the greater, the further away the column amplifiers are from one another, i.e. the amplified signals of the upper row of column amplifiers, on the one hand, and the amplified signals of the lower row of signal amplifiers, on the other hand, will systematically differ from one another. This difference becomes visible as a different brightness in the image. If the image of the one row of column amplifiers is now shown brighter, all odd columns are brighter than all even columns, or vice versa. This results in vertical brightness stripes or in a stripe pattern in the image.
In image sensors which are designed as color sensors, i.e. for example, are provided with a color filter arrangement with a Bayer pattern, blue-sensitive pixels and red-sensitive pixels lie alternately next to one another in adjacent columns. With a color sensor, the aforesaid stripe pattern therefore results in corresponding color differences in the image.